This application is the U.S. national phase of PCT application PCT/EP99/09530) filed Dec. 6, 1999 with a claim to the priority of German patent application 19859619.7 itself filed Dec. 23, 1998 and German patent application 19903936.9 itself filed Jan. 26, 1999.
The invention relates to an apparatus for contact-free guiding or treating a moving material web, in particular a paper or cardboard web or a metal or plastic foil, where the material web is supported on a gaseous fluid cushion according to the introductory clause of claim 1. Furthermore the invention relates to a coating apparatus that uses the apparatus according to the invention for supporting the material web, a dryer and a rewetting apparatus wherein the apparatus according to the invention is used to maintain moisture content of a material web at a certain level, and a web-storage system and web spreading apparatus for material webs.
In treatment systems for material webs it is often necessary to guide and support the web without contacting it; for example in coating equipment when the still-wet web must be supported on its coated side. To this end for example deflecting/guiding systems are known that are formed of a hollow body with an arcuate outer surface provided with nozzles through which compressed air moves outward from inside the hollow body. The web thus floats on a cushion of air above the outer support surface. German 2,252,574 describes a similar deflecting/guiding apparatus that has a chamber to which compressed air is fed and which has a gas-permeable wall whose outer surface is formed as the guide surface for the web. With this known deflecting/guiding apparatus the air cushion is several millimeters thick. The compressed air moves from inside through openings formed as nozzles so that large quantities of compressed air are needed and as a result there are substantial pressure losses at regions not covered by the web.
It is an object of the invention to improve an apparatus of the described type for guiding and treating a moving material web to ensure contact-free guiding of the web over the guide surface using and losing small amounts of support fluid.
This object is attained with the features of claim 1.
According to the invention the gas-permeable wall of the chamber, whose outer surface serves as guide surface for the web, is made of a metal-containing material whose open pores have an average diameter of less than 500 xcexcm, preferably less than 100 xcexcm, and in particular less than 20 xcexcm.
The use of the porous material for the guide surface makes it possible to slide the web on a fluid cushion less than 1 mm thick. Losses of the gaseous fluid on the regions not covered by the web are quite minimal. A further advantage is that a porous material is automatically cleaned by the gaseous fluid. Coating materials that drip on the guide surface are carried away by the produced fluid cushion without clogging the pores.
According to the preferred embodiment according to claim 1 the material for the gas-permeable wall is a porous composite of a thermosetting resin and metal, preferably a composite of a thermosetting resin and aluminum. The grain of such composites has nondirectionally distributed open pores that form branching passages through the material. The gaseous fluid flows very uniformly through the walls. In addition the material is of stable shape and is easy to work.
A material with a percentage of open pores on the outer surface of the gas-permeable wall of less than 20%, preferably less than 10%, has shown itself particularly ideal with respect to the low relationships and the pressure cushion formed.